by Kenneth W. Krause.
[Notable New Media]
Kenneth W. Krause is a contributing editor and “Science Watch” columnist for the Skeptical Inquirer. Formerly a contributing editor and books columnist for the Humanist, Kenneth contributes regularly to Skeptic as well. He may be contacted at firstname.lastname@example.org.
Could it be that Time magazine’s August 17, 2009 cover story announcing “The Myth About Exercise” misled millions of potential exercisers about the true relationship between physical activity (PA) and weight loss and maintenance? James Hill, professor of pediatrics and medicine at the University of Colorado Denver, and co-founder of the National Weight Control Registry and the America on the Move initiative, seems to think so.
In the spring of 2015, the National Academies’ Institute of Medicine convened a two-day workshop titled, “Physical Activity: Moving Toward Obesity Solutions.” Gathering preeminent experts in several relevant fields, the panel’s expressed purpose was to summarize the current science exploring “the impact of physical activity in the prevention and treatment of overweight and obesity and to highlight innovative strategies” for physical fitness. Their results were recently published by the National Academy of Sciences (IOM 2015).
We all understand that, in the limited context of weight loss and maintenance, those predisposed to excess adiposity cannot outrun, or out-exercise, a poor diet. And while it might take three minutes to consume a 560-calorie hamburger, for example, one would have to exercise forty-five to sixty minutes, depending on intensity, among other things, to burn it off. But does that necessarily mean, as the 2009 Time article implied, that PA is a trivial strategy in the difficult battle against an unhealthy bulge?
Not according to Hill, and not according to the science. Consider first the issue of “compensation.” Through many popular sources, including Time, we have been led to believe that those who increase their level of PA tend to compensate by either consuming more calories or increasing sedentary behavior.
But, as Hill reveals, a recent systematic review of 30 studies shows that, in most cases, exercisers did in fact not compensate with reductions in non-exercise PA (Washburn et al. 2014). Another study demonstrated that people who increased their PA tended not to completely compensate with increased caloric intake (Schubert et al. 2013). In summary, according to Hill, “the scientific literature indicates that when physical activity is added to a weight loss program, the majority of people do not compensate, at least not completely.” The “net result” of PA, in other words, is “a negative energy balance.”
Between 1960 and 2010, daily occupational (including housework) energy expenditure decreased by 120 calories per day, and more recent statistics suggest a further and continuing decline. “That is enough,” Hill argues, “to explain most of obesity.” Every study conducted on highly palatable, energy-dense diets, he continues, has demonstrated less weight gain when PA is added to improved nutrition—“even among people genetically susceptible to weight gain.”
Here, Doctor Hill draws our attentions to two important concepts. First, “metabolic flexibility” determines how efficiently our bodies can switch fuels. During a relative fasting state, a flexible metabolism can quickly suppress glucose oxidation and enhance fat oxidation, whereas an inflexible metabolism maintains “a blunted preference for fat oxidation” and remains unable to suppress the use of glucose. During an insulin-stimulated state, by contrast, the flexible person can suppress fat oxidation and increase her use of glucose, while the inflexible person is less capable of suppressing fat use and stimulating glucose oxidation.
Metabolic flexibility, Hill contends, is directly related to PA. While weight loss alone does not necessarily improve the situation, “[w]hen people stop moving, their metabolism loses its flexibility.” The resulting inflexibility, he reasons, renders people, including calorie-restricting dieters, “more susceptible to storing rather than burning fat.”
Second, some researchers believe that one can control personal energy balance far more efficiently by crossing a “threshold of PA.” Above that threshold is the “regulated zone,” and below it is the “unregulated zone.” In the former zone, we would expect to observe “a total compensation of energy intake with increased physical activity” and no weight change. In such cases, physical activity is “driving the bus” and “food is just along for the ride.” In the latter, unregulated zone, however, “as physical activity decreases, food intake actually increases.” Here, a tight coupling between PA and caloric intake collapses, and food is now “driving the bus.”
Hill suspects that decreasing PA “is the reason why most people today occupy the unregulated zone.” When people do lose weight, of course, their energy needs decrease. If a 220-pound person, for example, were to lose ten percent of his body weight, his energy demands would also plummet by roughly 170 to 250 calories per day. What’s the best solution? Fill that “energy gap,” Hill prescribes, with an increase in PA—which of course is far more sustainable than a decrease in energy intake.
Based on data from Hill’s National Weight Loss Registry, a paltry eight percent of the population will maintain a reduction in weight achieved by improved diet alone. Despite popular media manipulation and misinformation, most people who maintain weight loss do so through a combination of physical activity and diet. It is therefore extremely unlikely, Hill stresses, that any individual case of obesity can be resolved with improved nutrition alone, or that the obesity epidemic can be reversed without increasing PA in the broader population.
IOM (Institute of Medicine). 2015. Physical activity: moving toward obesity solutions: workshop summary. Washington, D.C.: The National Academies Press.
Schubert, M.M., B. Dresbow, S. Sabapathy, and M. Leveritt. 2013. Acute exercise and subsequent energy intake: a meta-analysis. Appetite 63:92-104.
Washburn, R.A., K. Lambourne, A.N. Szabo, et al. 2014. Does increased prescribed exercise alter non-exercise physical activity/energy expenditure in healthy adults? A systematic review. Clinical Obesity 4(1):1-20.